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Elite on the BBC Micro

Main loop: Main game loop (Part 4 of 6) (6502SP version)

Name: Main game loop (Part 4 of 6) [View in context] Type: Subroutine [Compare versions] Category: Main loop Summary: Potentially spawn a lone bounty hunter, a Thargoid, or up to four pirates Deep dive: Program flow of the main game loop Ship data blocks
This section covers the following: * Potentially spawn (35% chance) either a lone bounty hunter (a Cobra Mk III, Asp Mk II, Python or Fer-de-lance), a Thargoid, or a group of up to 4 pirates (a mix of Sidewinders, Mambas, Kraits, Adders, Geckos, Cobras Mk I and III, and Worms) * Also potentially spawn a Constrictor if this is the mission 1 endgame, or Thargoids if mission 2 is in progress
DEC EV \ Decrement EV, the extra vessels spawning delay, and BPL MLOOPS \ jump to MLOOPS if it is still positive, so we only \ do the following when the EV counter runs down INC EV \ EV is negative, so bump it up again, setting it back \ to 0 LDA TP \ Fetch bits 2 and 3 of TP, which contain the status of AND #%00001100 \ mission 2 CMP #%00001000 \ If bit 3 is set and bit 2 is clear, keep going to BNE nopl \ spawn a Thargoid as we are transporting the plans in \ mission 2 and the Thargoids are trying to stop us, \ otherwise jump to nopl to skip spawning a Thargoid JSR DORND \ Set A and X to random numbers CMP #200 \ If the random number in A < 200 (78% chance), jump to BCC nopl \ nopl to skip spawning a Thargoid .fothg2 JSR GTHG \ Call GTHG to spawn a Thargoid ship and a Thargon \ companion .nopl JSR DORND \ Set A and X to random numbers LDY gov \ If the government of this system is 0 (anarchy), jump BEQ LABEL_2 \ straight to LABEL_2 to start spawning pirates or a \ lone bounty hunter CMP #90 \ If the random number in A >= 90 (65% chance), jump to BCS MLOOPS \ MLOOPS to stop spawning (so there's a 35% chance of \ spawning pirates or a lone bounty hunter) AND #7 \ Reduce the random number in A to the range 0-7, and CMP gov \ if A is less than government of this system, jump BCC MLOOPS \ to MLOOPS to stop spawning (so safer governments with \ larger gov numbers have a greater chance of jumping \ out, which is another way of saying that more \ dangerous systems spawn pirates and bounty hunters \ more often) .LABEL_2 \ In the 6502 Second Processor version, the LABEL_2 \ label is actually ` (a backtick), but that doesn't \ compile in BeebAsm and it's pretty cryptic, so \ instead this version sticks with the label LABEL_2 \ from the cassette version \ Now to spawn a lone bounty hunter, a Thargoid or a \ group of pirates JSR Ze \ Call Ze to initialise INWK to a potentially hostile \ ship, and set A and X to random values CMP #100 \ If the random number in A >= 100 (61% chance), jump BCS mt1 \ to mt1 to spawn pirates, otherwise keep going to \ spawn a lone bounty hunter or a Thargoid INC EV \ Increase the extra vessels spawning counter, to \ prevent the next attempt to spawn extra vessels AND #3 \ Set A = random number in the range 0-3, which we \ will now use to determine the type of ship ADC #CYL2 \ Add A to #CYL2 (we know the C flag is clear as we \ passed through the BCS above), so A is now one of the \ lone bounty hunter ships, i.e. Cobra Mk III (pirate), \ Asp Mk II, Python (pirate) or Fer-de-lance TAY \ Copy the new ship type to Y JSR THERE \ Call THERE to see if we are in the Constrictor's \ system in mission 1 BCC NOCON \ If the C flag is clear then we are not in the \ Constrictor's system, so skip to NOCON LDA #%11111001 \ Set the AI flag of this ship so that it has E.C.M., STA INWK+32 \ has a very high aggression level of 28 out of 31, is \ hostile, and has AI enabled - nasty stuff! LDA TP \ Fetch bits 0 and 1 of TP, which contain the status of AND #%00000011 \ mission 1 LSR A \ Shift bit 0 into the C flag BCC NOCON \ If bit 0 is clear, skip to NOCON as mission 1 is not \ in progress ORA MANY+CON \ Bit 0 of A now contains bit 1 of TP, so this will be \ set if we have already completed mission 1, so this OR \ will be non-zero if we have either completed mission \ 1, or there is already a Constrictor in our local \ bubble of universe (in which case MANY+CON will be \ non-zero) BEQ YESCON \ If A = 0 then mission 1 is in progress, we haven't \ completed it yet, and there is no Constrictor in the \ vicinity, so jump to YESCON to spawn the Constrictor .NOCON LDA #%00000100 \ Set bit 2 of the NEWB flags and clear all other bits, STA NEWB \ so the ship we are about to spawn is hostile \ We now build the AI flag for this ship in A JSR DORND \ Set A and X to random numbers CMP #200 \ First, set the C flag if X >= 200 (22% chance) ROL A \ Set bit 0 of A to the C flag (i.e. there's a 22% \ chance of this ship having E.C.M.) ORA #%11000000 \ Set bits 6 and 7 of A, so the ship is hostile (bit 6) \ and has AI (bit 7) STA INWK+32 \ Store A in the AI flag of this ship TYA EQUB &2C \ Skip the next instruction by turning it into \ &2C &A9 &1F, or BIT &1FA9, which does nothing apart \ from affect the flags .YESCON LDA #CON \ If we jump straight here, we are in the mission 1 \ endgame and it's time to spawn the Constrictor, so \ set A to the Constrictor's type .focoug JSR NWSHP \ Spawn the new ship, whether it's a pirate, Thargoid, \ Cougar or Constrictor .mj1 JMP MLOOP \ Jump down to MLOOP, as we are done spawning ships .fothg LDA K%+6 \ Fetch the z_lo coordinate of the first ship in the K% AND #%00111110 \ block (i.e. the planet) and extract bits 1-5 BNE fothg2 \ If any of bits 1-5 are set (96.8% chance), jump up to \ fothg2 to spawn a Thargoid \ If we get here then we're going to spawn a Cougar, a \ very rare event indeed. How rare? Well, all the \ following have to happen in sequence: \ \ * Main loop iteration = 0 (1 in 256 iterations) \ * Skip asteroid spawning (87% chance) \ * Skip cop spawning (0.4% chance) \ * Skip Thargoid spawning (3.2% chance) \ \ so the chances of spawning a Cougar on any single main \ loop iteration are slim, to say the least LDA #18 \ Give the ship we're about to spawn a speed of 27 STA INWK+27 LDA #%01111001 \ Give it an E.C.M., and make it hostile and pretty STA INWK+32 \ aggressive (though don't give it AI) LDA #COU \ Set the ship type to a Cougar and jump up to focoug BNE focoug \ to spawn it .mt1 AND #3 \ It's time to spawn a group of pirates, so set A to a \ random number in the range 0-3, which will be the \ loop counter for spawning pirates below (so we will \ spawn 1-4 pirates) STA EV \ Delay further spawnings by this number STA XX13 \ Store the number in XX13, the pirate counter .mt3 JSR DORND \ Set A and X to random numbers STA T \ Set T to a random number JSR DORND \ Set A and X to random numbers AND T \ Set A to the AND of two random numbers, so each bit \ has 25% chance of being set which makes the chances \ of a smaller number higher AND #7 \ Reduce A to a random number in the range 0-7, though \ with a bigger chance of a smaller number in this range ADC #PACK \ #PACK is set to #SH3, the ship type for a Sidewinder, \ so this sets our new ship type to one of the pack \ hunters, namely a Sidewinder, Mamba, Krait, Adder, \ Gecko, Cobra Mk I, Worm or Cobra Mk III (pirate) JSR NWSHP \ Try adding a new ship of type A to the local bubble DEC XX13 \ Decrement the pirate counter BPL mt3 \ If we need more pirates, loop back up to mt3, \ otherwise we are done spawning, so fall through into \ the end of the main loop at MLOOP